Byzantium was an ancient Greek city, founded by colonists from Megara in 667 BC. After siding with Pescennius Niger against the victorious Septimius Severus the city was besieged and suffered extensive damage in AD 193-195. Byzantium was rebuilt by the now Roman Emperor Septimius Severus and quickly regained it's previous prosperity. The location of Byzantium attracted Constantine I, the Great who, in AD 330, refounded it as Nova Roma or Constantinoupolis (Constantinople) after a prophetic dream was said to have identified the location of the city.
Of course it didn't take a prophet to see that this location would play an important role as the crossing point between two continents (Europe and Asia), and later a magnet for Africa and others as well, in terms of commerce, culture, diplomacy and strategy. As a strategic position again, Constantinoupolis, was able to control the passageway between Asia and Europe, as well as the passage from the Mediterranean Sea to the Euxinos Pontus.